The present invention relates generally to the use of plasma polymerization to apply a very thin top coating of of a chemically resistant polymeric material over the surface of a polymeric substrate to protect the substrate from chemical attack by the electrolyte and also to preserve surface treatments performed on the substrate prior to deposit of the top coating. More particularly, the present disclosure relates to an improved membrane for an electrolytic cell and a method of preparation thereof utilizing plasma polymerization techniques to deposit a continuous top coating of approximately 100 to 2000 Angstroms on a substrate material generally a copolymeric cation exchange material having pendent sulfonic acid groups with polymers of tetrafluoroethylene or of an amide. The resultant coated membrane surfaces produced current efficiencies exceeding those of the untreated membranes or surface treated membranes prior to coating thereof due to the drastic reduction of the permeability of the membrane material with a minimal effect upon the bulk properties and the cell potential, while maintaining good lifetimes under corrosive conditions.
Electrochemical methods of manufacture are becoming ever increasingly important to the chemical industry due to their greater ecological acceptability, potential for energy conservation, and the resultant cost reductions possible. Therefore a great deal of research and development has been applied to the electrochemical processes and the hardware for these processes. One major element for the hardware aspect of the electrolytic system is the cation exchange membrane which separates the anode compartment from the cathode compartment within the electrolytic cell to provide a divided electrolytic cell for more efficient electrochemical production.
Presently the membrane having the greatest utility is one capable of usage in a chlorine and caustic (sodium hydroxide) cell since chlorine and caustic in this country are produced almost entirely electrolytically from aqueous solutions of sodium chloride. Chlorine and caustic are essential and large volume commodities which are basic chemicals required by all industrial societies and presently a large portion of their manufacture comes from the diaphragm-type electrolytic cells. In the diaphragm cell process, brine (sodium chloride solution) is fed continuously into the anode compartment and flows through the diaphragm usually made of asbestos, backed by a cathode. To minimize back migration of the hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has sodium chloride present. Hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution, containing caustic, unreacted sodium chloride and other impurities, generally has been concentrated and purified to obtain a marketable alkali metal hydroxide commodity and an alkali metal chloride which can be reused in a chlorine and caustic electrolytic cell for further production of alkali metal hydroxide. This is a serious drawback since the costs of this concentration purification process are rising rapidly.
With the advent of technological advances such as the dimensionally stable anode which permits ever narrowing gaps between the electrodes and the hydraulically impermeable cation exchange membrane, it has become readily apparent that the electrolytic cell will be more economical in the future. If the membrane can produce at high current efficiencies and withstand the anolyte solution which normally contains highly corrosive concentrations of free halide for extended periods of time, a significant purity and concentration increase in the end product may be possible with a given electrolytic cell thus saving secondary steps in such a process.
To date the membrane which seems to have the longest life time is a thin film of fluorinated copolymer having pendent sulfonic acid groups. Membranes of this general type are available from E. I. duPont deNemours & Co. under the trademark NAFION. It has been found that certain types of surface treatments of the NAFION-type membrane substrate will yield higher current efficiencies but generally with a corresponding higher cell potential and a reduction in the useful lifetime of the membrane in the corrosive surroundings of a chlorine and caustic electrolytic cell. It would therefore be very advantageous to provide an improved membrane which will have a substantially longer useful lifetime in addition to having the improved current efficiency characteristics of membranes currently for use in various types of electrolytic cells for electrochemical production.